Floppy disk drives are mechanisms for transferring data to and from floppy disks which have become virtually standardized media in the industry. The floppy disk is enclosed in a square envelope and has a central aperture which is used to position and then rotate the disk within the envelope.
Floppy disk drives are made to substantially standard outside dimensions, inasmuch as original equipment manufacturers which install them in their data processing systems desire to provide standard receptacles into which units from different manufacturers can be inserted. There are thus now "full-height" and "half-height" drives which, although they differ in internal component layout, are essentially interchangeable from the user's standpoint.
When a floppy disk is inserted in the receiving slot in the front panel of a drive, it moves into position in which the edge of the central aperture is in alignment with a rotatable spindle having an upper flange surface and a central recess. The spindle is driven by a motor directly or through a belt arrangment.
Although different mechanisms are used to initiate drive operation, including automatic actuators which respond to full insertion and lock-in of the disk envelope, a common and often preferred technique is to utilize a manual actuator to begin operation. Turning of a control lever at the front panel, for example, is utilized to generate a sequence of actions in which a pivotable arm carrying a spring-loaded positioning and clamping cone is moved down toward the disk. The cone is aligned with the central axis of the disk, as well as the rotatable spindle on the opposite side. As the cone enters the central aperture, compliant fingers or petals on the cone act against the edge of the central aperture to make the disk concentric with the central axis, so that the recording tracks on the disk are precisely placed and concentric about the central axis. In its final stage of downward movement, the cone clamps the disk against the upper flange of the spindle with a force dependent principally upon the final position of the cone arm and the characteristics of the compression spring that loads the cone. Downward movement of the cone arm is also utilized to control a switch which, after an appropriate delay, actuates the spindle motor and starts the floppy disk spinning within its envelope.
The so-called "half-height" floppy drive places a number of contraints on the designer because the same performance and interchangeability are required as with the much larger full height design. Accordingly, the principal components of the drive must be designed and packaged to be compatible with the smaller available volume. While this can be done quite readily with complex or precision made parts, the drives must also be cost competitive while meeting the needed precision and interchangeability requirements. This means that while extensive testing and adjustment can be done in theory, practical economic factors dictate that the number of parts should be kept to a minimum and that the number of adjustments must be simplified and minimized. Existing designs for positioning and adjusting the positioning cone mechanism are less than satisfactory in this respect. For size and cost considerations, they often employ a short pivotable arm mounted to one side of the floppy disk with a pivot spaced just outside the periphery of the floppy disk. An actuator forces the pivot arm downwardly toward the floppy disk so that the positioning cone at the end of the arm can be urged into registration with the drive spindle on the other side. With a relatively short pivot arm length, it is possible to utilize a stamped, sheet metal construction for cost reduction purposes. However, parts formed by these mass production techniques at suitably low cost have dimensional variations, so that adjustments must be provided for in the design. This is particularly true when the design is such that the misplacement of the positioning cone relative to the nominal central axis of the floppy disk, and the final engagement pressure of the positioning cone against the hub that is underneath, reflect a possible buildup of a number of tolerance variations. For example, using a rotatable control lever for actuating a cam which displaces the cone arm, the position of the control lever shaft can vary, as can the size or relationship of the cam mounted on the shaft. The position and shape of the pivotable cone arm as well as the position of the positioning cone relative to the pivot arm can also vary substantially. If the manufacturing variations all happen to fall to one side of the allowable tolerance ranges, the net result may well be that floppy disks are not interchangeable between drive units. In addition, the clamping of the disk to the spindle may either be so loose or so tight that the disk is not properly driven. The force with which the disk is pressed into driving engagement with the spindle is especially critical where the spindle is directly driven by the spindle motor. Such drive schemes are becoming prevalent in half-height disk drives to conserve space and to reduce the number of parts. In the absence of the torque multiplication furnished by a belt transmission, for example, the speed of direct drive motors is particularly sensitive to the disk clamping force. High force levels requiring drive torques easily produced by belt drives can stall a direct drive motor. Accordingly, there is a need for a mechanism that allows precise adjustment of the disk clamping force in a simple fashion both during production and in the field.